1. Field of the Invention
The present invention relates to a gear shift system for a vehicle as well as a control device and a control method for an automatic transmission, and particularly to a technique for controlling a gear ratio of an automatic transmission.
2. Description of the Background Art
A vehicle equipped with an automatic transmission has been known. In general, a gear ratio of the automatic transmission is determined according to an accelerator pedal position and a vehicle speed. For setting the gear ratio more finely, it is preferable to give consideration to running environment (a gradient of a road surface, a curvature of the road surface, a friction coefficient of the road surface, a degree of traffic jam and a type of the road) of the vehicle and the like as well as an accelerator pedal position and a vehicle speed.
Japanese Patent Laying-Open No. 9-126307 has disclosed a gear shift control device of an automatic transmission including a deceleration state determining unit determining a deceleration state of a vehicle, a deceleration gear setting unit for setting gears within gear selection ranges of different drive state parameters, respectively, based on the plurality of drive state parameters when deceleration determination is performed, and a gear shift control unit performing gear shift control by determining, as the gear, a lowest gear among the plurality of gears that are set by the deceleration gear setting unit.
According to the gear shift control device described in the above publication, the selection range of the gear for each parameter is different from those for the other parameters in the deceleration state. Therefore, such a structure is employed, e.g., that a second gear is selected for one parameter, but a third gear is selected as the lowest gear for another parameter. Thereby, it is possible to reduce the number of times that the second gear is finally selected, and the excessive engine braking can be avoided.
However, the gear shift control device disclosed in Japanese Patent Laying-Open No. 9-126307 finally selects the gear that is set based on one of the drive state parameters. Therefore, the gear may not be selected in view of mutual effects of the plurality of parameters. For example, the second gear may not be selected in the case where it is appropriate to select the third gear when consideration is given only to a road surface gradient, to select the third gear when consideration is given to only an accelerator pedal position and to select the second gear when consideration is given to both the road surface gradient and the accelerator pedal position. Further, the gear shift control device disclosed in Japanese Patent Laying-Open No. 9-126307 determines the gear according to a two-dimensional shift map using a vehicle speed, a throttle position and the like. Therefore, a parameter other than those used in the shift map cannot be reflected in the gear without difficulty. Therefore, there is a room for further improvement for setting more finely the gear, i.e., the gear ratio of the automatic transmission.